In recent years, flat panel displays such as liquid crystal displays (LCDs) are becoming widespread. In each of pixels provided in the row direction and the column direction in a display device such as a flat panel display, a transistor serving as a switching element, a liquid crystal element electrically connected to the transistor, and a capacitor connected to the liquid crystal element in parallel are provided.
As a semiconductor material of a semiconductor film of the transistor, a silicon semiconductor such as amorphous silicon or polysilicon (polycrystalline silicon) is generally used.
Metal oxides having semiconductor characteristics (hereinafter referred to as oxide semiconductors) can be used for semiconductor films in transistors. For example, techniques for forming transistors using zinc oxide or an In—Ga—Zn-based oxide semiconductor are disclosed (see Patent Documents 1 and 2).
A display device which includes a capacitor in which an oxide semiconductor film provided over the same surface as an oxide semiconductor film of a transistor and a pixel electrode connected to the transistor are provided to be separated from each other with a given interval in order to increase the aperture ratio is disclosed (see Patent Document 3).